1. Field of the Invention
This invention relates to zoom lenses, and, more particularly, to zoom lenses provided with a component of small refractive power movable along with a variator to effect a change in image magnification and which is made in part movable independently of the other to effect focusing.
2. Description of the Prior Art
In the past, for use as the focusing method for zoom lenses, impartment of independent movement to the front lens group, or the so-called front focusing method, has generally been well known.
For example, in the zoom lens comprising, from front to rear, a 1st lens group which remains stationary during zooming, a 2nd lens group movable for variation of the image magnification, a 3rd lens group movable for image shift compensation, and a 4th lens group or image forming system which is stationary during zooming, or the so-called 4-component mechanically compensated type zoom lens, when focusing from an infinitely distant object point to a close object point, the aforesaid 1st lens group is made to move axially. In this case, for a given finite object distance, the object point-to-image point relation of the aforesaid 1st lens group does not depend upon the focal length of the entire system, and, therefore, the amount of movement of the 1st lens group from the focusing position for the infinitely distant object takes a certain value no matter where the zooming position may be. This is the most important advantage of the focusing method by moving the front lens members. However, it has the following drawbacks also.
In the zoom lens of high zoom ratio with the longest focal length being relatively large, or the so-called telephoto zoom lens, upon consideration of good aberration correction, the total length of the entire lens system tends to become long, and the diameter of the front lens members also tends to become large. Moreover, for variation with focusing of aberrations being as reduced as possible, the construction and arrangement of the elements of the focusing lens group become complicated. Because of this, the weight of such lens group tends to become very heavy. With such zoom lens, when focusing is performed by moving the front lens group, there is need of a very large driving force. Further, to assure smoothness of the focusing operation, it is required that the mechanical mounting therefor be of satisfactory precision accuracy and durability.
Attempts have been made to overcome those drawbacks by dividing the focusing or 1st lens group into two parts, one of which is made movable for focusing. And many different proposals have been offered for such focusing method. By using this method, it is made relatively easy to reduce the total focusing movement and the weight of the movable part of the focusing lens group. In in application to auto-focus cameras, for example, however, the following drawback further arises.
For automatic focusing adjustment, the focusing control mechanism has to be operated with supply of a driving torque controlled by electrical signals, for example, from an electric motor lying in the interior of the camera. When the lens group to be driven is located farthest off from the rear side, or the drive source within the camera housing, therefore, the structure of transmission of the driving torque becomes complicated. For this reason, the lens system for use in auto-focus cameras is desired to fulfill the following requirements:
(a) the weight of the focusing lens group is light; and
(b) the focusing lens group lies near the rear side.
The requirements described above appear such that any of the methods other than the front focusing method suffices. For example, in the case of the above-cited 4-component mechanically compensated zoom lens, the image forming or 4th lens group may be made movable in part or as a whole to effect focusing. As focusing starts from the position for an infinitely distant object, the required amount of movement of the focusing lens group is caused to change depending upon the focal length of the entire system, thus giving rise to an alternate problem. In general, such required amount of movement is lessened in the wide angle position, and increased in the telephoto position, the ratio being proportional to almost the 2nd power of the zoom ratio.
The use of such differential focusing movements with the focal length of the entire system in the zoom lens, when applied as the objective lens of, for example, auto-focus cameras, leads to the necessity of designing the operating mechanism in such a way that in the telephoto positions where the total focusing movement is longer, the focusing lens members are driven to move at a faster speed than that when in the wide angle positions, while maintaining high accuracy of adjustment control at any zooming station.
The fact that the adjustment in position of the focusing lens group is differentiated by the zooming position is not advantageous when considered simplification of the structure of the mechanism.
In order that the difference between the required amounts of focusing movement in the wide angle and telephoto positions is moderated by any value however small it may be, it should be now considered that the focusing lens group is made movable when zooming.
This can take either of the following two main forms:
(1) Since the focusing lens group is made to move during zooming, the resultant change in the relative position of the focusing lens group causes the image magnification of the focusing lens group to change, thereby the difference between the speeds of focusing movement in the wide angle and telephoto positions is reduced.
(2) The focusing lens group is constructed with a plurality of subgroups which are made movable independently of each other when zooming, so that the focusing lens group itself changes its refractive power, thereby the difference between the speeds of focusing movement in the wide angle and telephoto positions is reduced.
Since, however, both of the means are to move the focusing lens group during zooming, as a matter of course, the paths of movement of the other zoom groups are caused to change from those which would otherwise be made when the focusing lens group is held stationary during zooming, leaving a high possibility that a solution will not be found for movement of that zoom group which is for compensating for the image shift, or of further increasing the refractive power of the magnification varying lens group to obtain a desired zoom ratio.
It is generally proven that the concurrent impartment of a movement into the focusing lens group with zooming for the purpose of reducing the difference between the required amounts of movement in the wide angle and telephoto positions results in a large effect of reverse change of the magnification by that focusing movement, and is not always preferable when trying to achieve a high range of variation of the image magnification.
As the zoom lens having the focusing provision in one of the lens groups which lie in rear of the variator, mention may be made of those disclosed in Japanese Laid-Open patent application Nos. Sho 56-165106 and Sho 56-165107.